PORIFERA [sponges]. 25 



Sub-Class II. Demospongi.e {demos, multitude), including all High Cases 

 sponges other than Calcarea and Hexactinellida. Table Cases 



Grade I. Tetractinellida {tetra, four ; aktis, ray). Four-Ray i and 2 a, b. 

 or Anchor Sponges. Siliceous spicules typically with four axes and 

 four rays ; also certain fleshy sponges (Carnosa) are included here. 



Grade II. Monaxonida {moms, single ; axon, axis). Monaxon 

 Sponges. Siliceous spicules rod- or pin-shaped. 



Grade III. Keratosa {Jceras, horn). Horny Sponges. Skeleton 

 of horny fibre. 



Grade IV. MYXOSPONfUDA (wy.m, slime). Slime Sponges. With- 

 out a skeleton. 



Class I. CALCAREA [Calcareous Sponges]. 



The Calcareous Sponges form a comparatively small group, only Table Case 2a. 

 about 2<i(» recent species being known. They live for the most part 

 in shallow water, and prefer shady sheltered localities. 



The skeleton is composed of spicules of carbonate of Imie, which 

 are either separate, or, in a few instances, fused into a solid frame- 

 work. The spicules are either three-rayed, four-rayed, needle-shaped, 

 or, in one instance, spherulitic. 



The AsconidcB, which are the simplest of all sponges, are formed 

 of thin-walled branching tubes, lined with collar-cells throughout 

 their inner surface. The tubular branches may be separate (Leuco- 

 solenia), or may join to form a network {Glathrina). 



In the Syconid(c the collar-cells are restricted to " radial tubes " 

 surrounding a central cavity devoid of collar-cells. 



The beautiful Ciliated Sycon {Sycon cUiatiun) is one of the 

 commonest sponges round our coasts, where it is found attached to 

 rocks and seaweeds. This sponge, which is usually about an inch in 

 height, has the form of a little oval white sac with a silvery crown of 

 spicules round the orifice (oscule). The crown of spicules is fully 

 expanded when currents are passing through, but otherwise it is 

 closed. The wall of the sac is formed of horizontally arranged 

 closely-packed tubes each opening by a comparatively wide aperture 

 into the vertical central cavity, but ending blindly on the outer 

 surface of the sponge. Currents pass through microscopic orifices 

 in the walls of the radial tubes into the interior of those tubes and 

 into the central cavity, and finally leave through the oscule. 



A very large specimen of a Sycon Sponge, over eight inches in 

 length, from Poole Harbour, is exhibited in the Case. 



